EP1084327 describes a control method for feeding cooling air from a compressor to the stator and rotor blades of a given turbine stage. The cooling air flows through the stator blades and the stator inner ring into the rotor of the same stage, and finally through the rotor blades. A valve for regulating cooling airflow upstream from the stator is operated by a control unit, which operates on the basis of a feedback signal from a pressure sensor at the valve outlet, and, in the control method described, adjusts cooling airflow to maintain a predetermined pressure signal from the sensor.
This control strategy has the drawback of apparently not taking into account the power output or turbine load, and of failing to accurately adjust cooling air consumption. For example, in the event of excessive cooling air consumption, at high loads the reduction in combustion airflow could increase temperature in the combustion chamber and therefore nitric oxide emissions.
Conversely, an excessive reduction in cooling air consumption at low loads could cause a temperature drop in the combustion chamber and therefore an increase in carbon monoxide emissions and particulate.